This invention relates to a nonreversible circuit device for use in a terminal unit for transmission and reception in a radio communication system and, in particular, to a dual-band nonreversible circuit device operable in two different frequency bands specific to two different radio communication systems.
In recent years, technology has made remarkable progress in the field of radio communication. In many countries in the world, various radio communication systems are working and offered to users. However, those radio communication systems have different frequency bands assigned thereto. In order to enjoy the services of the various radio communication systems, it is necessary to use different terminal units for trans-mission and/or reception in the different frequency bands. Such use of the different terminal units is inconvenient and troublesome. In view of the above, consideration has recently made with respect to the necessity of a so-called dual-band terminal unit which is operable in two different frequency bands for two of the various radio communication systems.
The terminal unit typically includes a transmitter-receiver branching circuit for connecting a reception amplifier and a transmission amplifier to a common antenna and for isolating them from each other. The transmitter-receiver branching circuit generally includes components which are relatively large in size.
The transmitter-receiver branching circuit comprises a nonreversible circuit device. As an example of such a nonreversible circuit device, there is known a distributed-constant nonreversible circuit device comprising a magnet for generating a magnetic field, a set of center conductors, each serving as a signal path, a ferrite plate for providing the signal path with a unidirectional characteristic, and a ground conductor plate.
An improved nonreversible circuit device comprising two sets of center conductors arranged in a single magnetic circuit is disclosed, for example, in Japanese Unexamined Patent Publications Nos. 58-85609 (85609/1983) (Reference 1) and 4-345201 (345201/1992) (Reference 2).
In Reference 1, the two sets of center conductors are connected in cascade with each other in one-to-one correspondence and grounded in the single magnetic circuit so as to increase an inductance without increasing the size of the device. Thus, a small-sized high-performance nonreversible circuit device is obtained.
In Reference 2, the two sets of center conductors are connected in parallel with each other in one-to-one correspondence and grounded in the single magnetic circuit so as to decrease an inductance component and a d.c. resistance in a high-frequency region without increasing the size of the device. Thus, a small-sized nonreversible circuit device having excellent high-frequency characteristics is obtained.
It is noted here that the nonreversible circuit device described in each of References 1 and 2 is operable at a single operation frequency.
In order to use a single terminal unit in common in two different radio communication systems of two different frequencies, it has been a practice that the single terminal unit includes two transmitter-receiver branching circuits corresponding to the different frequencies. In addition, each of the transmitter-receiver branching circuits includes components which are relatively large in size as described in the foregoing. As a result, the terminal unit inevitably becomes bulky as a whole.
In view of achieving portability, however, it is desirable that the terminal unit is small in size and light in weight even in the case where the terminal unit is to be used in common in two different radio communication systems.
Reduction in size and weight of the terminal unit will be achieved if a single transmitter-receiver branching circuit is selectively operable at two different frequencies. In this event, the nonreversible circuit device is required to deal with the two different frequencies.
However, the nonreversible circuit device described in each of References 1 and 2 is operable at the single operation frequency. Therefore, for use in the two different radio communication systems of the two different frequencies, the terminal unit must include two nonreversible circuit devices corresponding to the two different frequencies. Thus, the nonreversible circuit device described in each of References 1 and 2 can not contribute to the reduction in size and weight of the terminal unit.